Many portable devices have been developed in which optical sensors are used to detect variation in blood flow through arteries or blood volume in subcutaneous tissue. Applications include the monitoring of heart rate, glucose level, apnea, respiratory stress, and other physiological conditions. The optical sensors often comprise one or more light sources that illuminate a targeted portion of the human body and one or more associated optical detectors that receive a portion of the optical energy emitted by the light sources.
There are two basic types of such arrangements. In transmissive sensor arrangements, a relatively thin portion of the body such as the tip of the finger or the earlobe is positioned between a light source and a photo detector. Light that passes through the body tissue impinges on the photo detector resulting in an electrical signal that is synchronized to each heartbeat. In reflective sensor arrangements, a sensor that includes one or more light sources located in spaced apart juxtaposition with a photo detector is positioned against a targeted area of the body. Optical energy emitted by the light sources passes through the skin of the targeted tissue region, is scattered, partially absorbed, and is reflected by blood flowing through arteries and other vascular structure. The reflected optical energy is in effect modulated in accordance with blood flow in the targeted area and detected by the photo detector. The detected reflection can then be used to produce a signal pulse that is indicative of a physiological parameter such as a heartbeat. In both transmissive and reflective arrangements, the signal produced by the photo detectors is processed to display or otherwise provide a real-time indication of the monitored physiological parameter.
One area of growing interest in the use of physiological monitors is with respect to personal wellness and/or physical exercise for purposes of fitness training, weight loss, or monitoring general health. Technological advances relating to optical sensors, signal processing, and display devices have made it possible to realize small, light-weight physiological monitors that can be embodied as devices that may be comfortably worn by a user. Such wearable devices may include, for example, wrist watches, bracelets, and arm bands.
Providing physiological monitors for wellness and physical exercise applications is subject to numerous design and manufacturing considerations. For example, the electronic circuitry for processing the signal produced by the photo detector and displaying an indication of the monitored parameter must operate at a low power level to provide adequate battery life while simultaneously providing sufficient accuracy. Constraints relating to the physical design of such monitors are not limited to the challenges of packaging the electronics and display units in an arrangement that can be easily and comfortably worn by a user. Special considerations and constraints are present with respect to incorporation of the optical sensor. For example, the light sources and photodiode of the optical sensor must be optically isolated from one another. Otherwise, the photo detector will receive optical energy that is not modulated by a user's heartbeat, which can result in an unwarranted increase in electrical design requirements and/or seriously affect monitoring accuracy and power requirements. Similarly, optimal performance requires that the optical sensor be firmly positioned against the user's skin so that light emitted by a light source may pass through the skin and, additionally, so that ambient light does not reach an associated photo detector. Firmly positioning the optical sensor against the user's skin also is important with respect to preventing movement of the sensor that can affect the accuracy of the monitoring device and/or interrupt its operation. Additionally, the optical sensor should be securely retained by the monitoring device to maintain physical integrity and facilitate satisfactory waterproofing of the entire monitor.
Because of the above mentioned design and manufacturing considerations, as well as others that are known to designers and manufacturers, a need exists for improved techniques for incorporating optical sensor arrangements in physiological monitoring devices. Moreover, improved device and techniques are needed to ensure the accuracy, reliability, and durability of such devices.